1. Field of the Invention
The present invention relates to a delivery system having a self-expanding braided stent intended for implantation into a blood vessel, with an outer sleeve, which has a distal end and a proximal end and in which an inner sleeve is arranged that is displaceable relative to the outer sleeve and protrudes, with a handling portion, from the proximal end of the outer sleeve, and with a tip, which is arranged at the distal end and is securely connected to a stent support on which the braided stent is arranged in its loaded state.
2. Related Prior Art
A delivery system of this kind is known commercially to the applicant of the present invention.
A stent is understood as a radially expandable endoprosthesis, which is a typical intravascular implant that is implanted via a lumen and is radially enlarged expanded after it has been introduced percutaneously. Stents are used, for example, to strengthen blood vessels and, in the vascular system, to prevent restenosis after angioplasty. In addition, they are also implanted in arteries for the treatment of aneurysms.
Such stents can be self-expanding or can be expanded by a radial force applied from the inside, for example when they are fitted on a balloon.
A very wide variety of stents are used depending on the type of application. The present invention is concerned with the application of what are called braided stents, such as are known, for example, from DE 197 50 97 A1 or DE 103 35 649 A1.
A braided stent is a metal stent that is produced by what is called a plain weaving technique. It is composed of a hollow body, which can stretch in the longitudinal direction and whose jacket is a braid made up of a multiplicity of filament-like elements which, in the expanded state of the braided stent, intersect a plane, perpendicular to the longitudinal direction, at a braid angle. A braided stent undergoes a considerable change in length when stretched, this change in length being all the greater the greater the original diameter and the smaller the original braid angle.
For implantation, a braided stent of this type is stored in an elongate configuration in what is called a delivery system or applicator, the latter being introduced percutaneously into the body at a suitable location, for example the femoral artery, and being guided through a lumen as far as the vessel where the stent is to be released. The delivery system and the stent are often provided with X-ray markers with which the positioning and the release of the stent can be monitored in situ.
In stents that experience no change or only a very slight change in length when released, the position of the implantable stent can be verified in this way without any difficulty, for which reason there are also many different delivery systems available for stents of this type that only expand radially.
In braided stents, however, a problem that arises is that they are very much shortened when released. At a braid angle of 40°, for example, the ratio between the stent length in the loaded state in the delivery system and the stent length in the expanded state, in other words the free state, is 1.5:1, for example. If the braid angle α is smaller, this ratio increases still further, and, at a braid angle of α=10°, the ratio can even be 4:1 to 6:1. It should be noted here that the shortening in length is of course also dependent on the diameter of the stent in the loaded state and on the diameter of the stent in the released state.
Braided stents are therefore extremely extensible and, in their elongated state, they as it were store mass which, upon contraction of the stents, ensures a compact and stiff functional area, as is explained in detail in aforementioned DE 103 35 649 A1.
Braided stents, in which this shortening has to be taken into account, have hitherto been releasable in the blood vessel only with a low level of positioning accuracy. However, the positioning of a stent at the desired location in the blood vessel is a critical factor that determines the effect of the stents and the success of the medical intervention. The area in the blood vessel where the stent is to be expanded is usually accessible to the physician only with difficulty.
For example, the stent has to be positioned in the area of an organ or, in the case of the carotid artery, in the region of the cerebral arteries, where in some cases only a relatively short stent has to be put in place. However, because of the considerable shortening that takes place upon release of a braided stent, conventional systems require a substantial length of release, as is the case, for example, in the system available on the market.
In the known delivery system, the braided stent in the loaded state, that is to say in the extremely elongated form, lies proximally from the tip and rests on the stent support, which is connected securely to the inner sleeve. The stent is pressed radially inward by the outer sleeve and is held in its elongated form.
To deploy the braided stent, the inner sleeve is held secure, and with it the stent support and the tip connected to the latter, and it must not be moved during the release. The outer sleeve is then pulled back carefully from the tip, such that the stent is released gradually. As the stent comes free, it contracts in the direction of the outer sleeve and thus moves away from the tip, which remains fixed in position relative to the vessel by the inner sleeve. As the outer sleeve is pulled back, the gradually released and radially expanding stent, contracting in the longitudinal direction, follows the outer sleeve until a portion of the braided stent has been released which is such that the latter bears securely against the inner wall of the vessel.
All of this means, however, that the tip of the delivery system has to be displaced distally far beyond the intended site of release of the braided stent in order to permit a correct placement. In the known delivery system, this amounts to several centimeters, which leads to a length of release that cannot be tolerated, especially in the case of small braid angles. Particularly in cases where a relatively short stent has to be placed in proximity to an organ or to the cerebral arteries, there is a danger that the further advanced tip will cause damage to the organ or the cerebral artery.
A further disadvantage of the known delivery system is seen in the fact that the position of the braided stent in the vessel can be influenced only with difficulty by manipulation of the outer sleeve. If the operating surgeon has pushed the delivery system in too far or not far enough, he can no longer influence the final position of the braided stent in the known delivery system. If he finds that the braided stent is released too far above or below the planned site of deployment, he has to pull the braided stent in again, which, in the known delivery system, can only be done in emergency situations, and not more than twice. To “reload” the braided stent into the delivery system, the outer sleeve is now held secure, while the inner sleeve is carefully pulled back. In the known delivery system, however, this reloading is only possible as long as not more than 50% of the braided stent has been released.
In the known delivery system, therefore, one disadvantage is that braided stents in particular with small braid angles cannot be positioned close enough to organs or cerebral arteries, etc., while a further disadvantage is that exact positioning is extremely difficult.